1. Field of the Invention
The invention relates to the field of power line communications and, more particularly, transceivers enabling such communications.
2. Prior Art
U.S. Pat. No. 4,918,690 describes a system for providing sensing bidirectional communications and control. Communications between a plurality of cells enables, for example, control for switching and the like. The present invention provides a transceiver for power line communications which may be used with the system described in U.S. Pat. No. 4,918,690.
A major problem with power line communications is background noise including impulse noise. This noise originates not only from the power source and distribution network, but also from the loads. The noise is not constant with respect to time and moreover, it varies from place to place in a power distribution network. A theoretical analysis of impulse noise, particularly for a twisted pair, is described in "Errors-and-Erasures Coding to Combat Impulse Noise on Digital Subscriber Loops", IEEE Transactions on Communications, Vol. 38, No. 8, August 1990, beginning at page 1145. The present invention, as will be seen, employs snubbing or blanking to eradicate noise impulses. Applicant is not aware of this technique being applied to a digital system. Snubbing or blanking has been used in connection with radio receivers and its use is described in U.S. Pat. No. 4,124,819.
One commercially available system "X-10" provides sensing, control and communications over power lines. Coded patterns are transmitted twice, once in true form and a second time in complementary form on a modulated carrier. This technique provides only limited immunity to power line noise. One problem with this system and other systems is that the carrier detection threshold level is fixed. In selecting a threshold level for such a system, the level must be relatively high to provide some immunity from expected noise. This limits the operation of the system, for instance, where both the noise and signal are low. As will be seen, the present invention provides an adaptive carrier detection level.
In some communications systems, it is known to transmit information redundantly on two channels and then to select the channel with the strongest signal at the receiver. This technique is used in wireless microphone systems. As will be seen, the present invention uses two channels, however, selection between one or the other (or both) channels is based on noise levels, not signal strength.
Data is often encoded with error correcting codes (ECC) such as Hamming codes to permit the detection and correction of errors. In the present invention, this coding is extended through use of a "hint" signal to locate possible errors. The hint signal is generated from an impulse noise detection circuit.
Another technique sometimes employed in power line communications is to use a spread spectrum. Once such power line communications apparatus is disclosed in U.S. Pat. No. 4,815,106.